Virtual machine monitor configured to support latency sensitive virtual machines

ABSTRACT

A host computer has a virtualization software that supports execution of a plurality of virtual machines, where the virtualization software includes a virtual machine monitor for each of the virtual machines, and where each virtual machine monitor emulates a virtual central processing unit (CPU) for a corresponding virtual machine. A virtual machine monitor halts execution of a virtual CPU of a virtual machine by receiving a first halt instruction from a corresponding virtual machine and determining whether the virtual machine is latency sensitive. If the virtual machine is latency sensitive, then a second halt instruction is issued from the virtual machine monitor to halt a physical CPU on which the virtual CPU executes. If the virtual machine is not latency sensitive, then a system call to a kernel executing on the host computer is executed to indicate to the kernel that the virtual CPU is in an idle state.

CROSS REFERENCE TO OTHER APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/870,143, entitled “TECHNIQUES TO SUPPORT HIGHLY LATENCY SENSITIVEVMs,” filed Aug. 26, 2013, the contents of which is incorporated hereinby reference. This application is related to: U.S. patent applicationSer. No. ______, entitled “CPU Scheduler Configured to Support LatencySensitive Virtual Machines” (Attorney Docket No. B487.01), filed Aug.25, 2014; U.S. patent application Ser. No. ______, entitled “NetworkingStack of Virtualization Software Configured to Support Latency SensitiveVirtual Machines” (Attorney Docket No. B487.03), filed Aug. 25, 2014;and U.S. patent application Ser. No. ______, entitled “Pass-throughNetwork Interface Controller Configured to Support Latency SensitiveVirtual Machines” (Attorney Docket No. B487.04), filed Aug. 25, 2014,the entire contents of which are incorporated herein by reference.

BACKGROUND

Applications characterized as “latency sensitive” are, typically, highlysusceptible to execution delays and jitter (i.e., unpredictability)introduced by the computing environment in which these applications run.Examples of latency sensitive applications include financial tradingsystems, which usually require split-second response time whenperforming functions such as pricing securities or executing andsettling trades.

One source of execution delay is the system kernel, specifically, thekernel scheduler that runs therein. The scheduling and de-scheduling ofsystem and user tasks contributes to latency in a number of ways. First,tasks that are scheduled for execution may spend a non-trivial amount oftime in a queue for a particular central processing unit (CPU) beforethe tasks are dispatched for execution on that CPU. Further, the tasks,once executing, may be preempted by higher priority tasks.

When a virtual CPU (which emulates a physical CPU for a virtual machine)is halted by a virtual machine's guest operating system, the executionof the virtual CPU is, in turn, halted on a physical CPU. This isusually achieved by the kernel scheduler preempting the execution of thevirtual CPU on some physical processor, and queueing the taskcorresponding to the virtual CPU for later execution. When the virtualCPU becomes ready to run again, the virtual CPU task is typically placedinto a run queue of the same or other CPU for subsequent execution onthat CPU. The intervention by the kernel scheduler to both preempt thevirtual CPU and to requeue the virtual CPU when the virtual CPU becomesready to run is a source of latency, which can cause problems forlatency sensitive applications executing on the preempted virtual CPU.

SUMMARY

A method of halting execution of a virtual CPU of a virtual machine on ahost computer is provided, where the host computer has a virtualizationsoftware that supports execution of a plurality of virtual machines, andwhere the virtualization software includes a virtual machine monitor foreach of the virtual machines, each virtual machine monitor emulating avirtual central processing unit (CPU) for a corresponding virtualmachine. The method comprises the steps of receiving a first haltinstruction at a virtual machine monitor from a corresponding virtualmachine and determining, by the virtual machine monitor, whether thevirtual machine is latency sensitive. The method further comprises thestep of, if the virtual machine is latency sensitive, issuing from thevirtual machine monitor a second halt instruction to halt a physical CPUon which the virtual CPU executes. The method further comprises the stepof, if the virtual machine is not latency sensitive, then executing asystem call to a kernel executing on the host computer to indicate tothe kernel that the virtual CPU is in an idle state.

Further embodiments provide a non-transitory computer-readable mediumthat includes instructions that, when executed, enable a host computerto implement one or more aspects of the above method, as well as avirtualized computing system that includes a host computer, a kernelscheduler, and a virtual machine monitor that is configured to implementone or more aspects of the above method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting a virtualized computing environment inwhich one or more embodiments may be implemented.

FIG. 2 is a block diagram that depicts a table for storing latencysensitivity information, according to embodiments.

FIG. 3 is a diagram that depicts the processing of a halt instructionissued by a highly latency sensitive virtual machine, according to oneor more embodiments.

FIG. 4 is a flow diagram depicting a method for halting a virtual CPU ofa highly latency sensitive virtual machine, according to one or moreembodiments.

DETAILED DESCRIPTION

FIG. 1 depicts a virtualized computing environment in which one or moreembodiments may be implemented. As shown, the computing environmentincludes a host computer 100 and a virtual machine (VM) managementserver 150. VM management server 150 communicates with host computer 100over a local connection or, alternatively, over a remote networkconnection.

Host computer 100 is, in embodiments, a general-purpose computer thatsupports the execution of an operating system and one more applicationprograms therein. In order to execute the various components thatcomprise a virtualized computing platform, host computer 100 istypically a server class computer. However, host computer 100 may alsobe a desktop or laptop computer.

As shown in FIG. 1, host computer 100 is logically divided into threecomponents. First, execution space 120 supports the execution ofuser-level (i.e., non-kernel level) programs. User-level programs arenon-privileged, meaning that they cannot perform certain privilegedfunctions, such as executing privileged instructions or accessingcertain protected regions of system memory. Among the programs thatexecution space 120 supports are virtual machines.

Virtual machines are software implementations of physical computingdevices and execute programs much like a physical computer. Inembodiments, a virtual machine implements, in software, a computingplatform that supports the execution of software applications under thecontrol of a guest operating system (OS). As such, virtual machinestypically emulate a particular computing architecture. In FIG. 1,execution space 120 includes VMs 110 ₁-110 _(N). Each VM 110 shownsupports the execution of one or more applications 111, each of whichexecutes under the control of a particular guest OS 112. Applications111 are user-level (non-kernel) programs, such as, for example, wordprocessors or spreadsheet programs. Each of the depicted guest OS′ 112may be one of the well-known commodity operating systems, such as any ofthe versions of the Windows® operating system from Microsoft Corp., theLinux® operating system, or MacOS® X from Apple, Inc. It should be notedthat the applications and guest OS′ may vary from one VM to another.Thus, applications 111 ₁ in VM 110 ₁ may include Microsoft's Word® andExcel® applications running under the control of Windows® 7 as guest OS112 ₁. By contrast, applications 111 _(N) in VM 110 _(N) may include theSafari® web browser running under the control of MacOS® X as guest OS112 _(N). As shown in FIG. 1, each of VMs 110 ₁-110 _(N) communicateswith a hypervisor component, referred to herein as hypervisor 130.

Hypervisor 130, as depicted in FIG. 1, provides the operating systemplatform for running processes on computer host 100. Hypervisor 130controls all hardware devices within computer host 100 and managessystem resources for all applications running therein. Among the corefunctions that hypervisor 130 provides are console services, file systemservices, device drivers, and resource scheduling. Further, hypervisor130 implements software components that provide for the instantiation ofone or more virtual machines on the host computer.

As depicted in the embodiment of FIG. 1, hypervisor 130 includes virtualmachine monitors (VMMs) 131 ₁-131 _(N). Each VMM 131 corresponds to anexecuting VM 110. Thus, VMM 131 ₁ corresponds to VM 110 ₁, VMM 131 ₂corresponds to VM 110 ₂, and so on. Each VMM 131 is a software layerthat provides a virtual hardware platform to the guest OS for thecorresponding virtual machine. It is through a particular VMM 131 that acorresponding VM accesses services provided by the kernel component ofhypervisor 130 (shown in FIG. 1 as kernel 136). Among the functionscarried out by kernel 136 are memory management, providing networkingand storage stacks, and process scheduling.

Each VMM 131 in FIG. 1 implements a virtual hardware platform for thecorresponding VM 110. Among the components of the implemented virtualhardware platform are one or more virtual central processing units (orVCPUs) 125. In FIG. 1, VMM 131 ₁ implements a VCPU 125 ₁, VMM 131 ₂implements VCPU 125 ₂, and so on. Each VCPU 125 appears as a physicalCPU from the standpoint of the applications 111 and the guest OS 112that run in the corresponding VM 110. In this way, a virtualized guestoperating system that runs within a virtual machine may schedule anddispatch processes for execution on one or more virtual CPUs in the sameway that an operating system that runs directly on a computer host(i.e., in a non-virtualized manner) schedules and dispatches processesfor execution on physical CPUs. However, from the standpoint ofhypervisor 130 (which, in typical embodiments, executes directly oncomputer host 100), each VCPU 125 is a process to be scheduled anddispatched on a physical CPU of computer host 100.

The guest operating systems executing in the virtual machines thatcorrespond to the VMMs (e.g., Windows or Linux) may determine that thereare no longer any tasks to run. For example, a virtual machine may haveexecuting therein a spreadsheet or word processing application. Whenthose applications are terminated, and no other applications arestarted, the guest operating system may determine that, in order to savepower, one or more CPUs (i.e., virtual CPUs for the correspondingvirtual machine) should be quiesced. In order to quiesce the virtualCPUs, the guest operating system may execute a “halt” instruction (e.g.,the HLT instruction that is part of the x86 architecture). In othercases, the guest operating system may issue a monitored wait (e.g.,MWAIT) instruction, where the CPU halts processing at a predefinedinstruction address. In either case, the CPU ceases processing andenters an idle state. In one or more embodiments, the CPU returns fromthe idle state when an interrupt to the CPU is received. The VMMs (suchas VMMs 131) emulate the quiescing (or halting) of the virtual CPUs suchthat the halted virtual CPUs appear as quiesced physical CPUs to theguest operating system executing in a corresponding virtual machine.

In one or more embodiments, kernel 136 serves as a liaison between VMs110 and the physical hardware of computer host 100. Kernel 136 is acentral operating system component, and executes directly on host 100.In embodiments, kernel 136 allocates memory, schedules access tophysical CPUs, and manages access to physical hardware devices connectedto computer host 100.

Kernel 136 also includes a kernel scheduler 135. Kernel scheduler 135 isresponsible for scheduling tasks for execution on the physical CPUs ofcomputer host 100. It should be noted that all tasks that execute oncomputer host 100 share its underlying hardware resources. This includesrandom access memory, external storage, and processing time on thephysical CPUs. Thus, the tasks that kernel scheduler 135 schedules forprocessing include VCPUs 125 (which are the virtual CPUs of executingVMs), as well as non-virtualized standalone applications, kernelthreads, and interrupt handlers. In embodiments, kernel scheduler 135maintains a separate run queue for each physical CPU (not shown). Thatis, kernel scheduler 135 schedules a process for execution on aparticular physical CPU by placing that process in the run queue forthat particular physical CPU. Typically, a physical CPU only selectsprocesses from its own queue for execution thereon.

However, it is possible for one physical CPU to become idle while otherphysical CPUs have jobs waiting in their corresponding run queues. Thus,periodically, kernel scheduler 135 rebalances the queues. That is, ifthe run queue of a particular physical CPU contains a large number oftasks, kernel scheduler 135 moves some of the tasks therein to runqueues of physical CPUs that are less busy.

FIG. 1 also depicts hardware platform 140, which is another component ofcomputer host 100. Hardware platform 140 includes all physical devices,channels, and adapters of computer host 100. Hardware platform 140includes network adapters (not shown), for network communication, aswell as host bus adapters (HBAs) (not shown), which enable communicationwith external storage devices. In addition, hardware platform 140includes the physical CPUs (referred to herein as PCPUs) of computerhost 100. As shown in FIG. 1, computer host 100 has M PCPUs 145 ₁-145_(m). As shown by the figure, kernel scheduler 135 schedules anddispatches tasks for execution on one or more PCPUs 145.

Hardware platform 140 also includes a random access memory (RAM) 141,which, among other things, stores programs currently in execution, aswell as data required for such programs. Moreover, the aforementionedrun queues that kernel scheduler 135 maintains for each PCPU aretypically maintained in RAM 141.

In order to support the configuration, identification, and processingchanges needed for executing highly latency sensitive virtual machines,the embodiment depicted in FIG. 1 includes a VM management server 150.VM management server 150 is, in embodiments, a server applicationexecuting either within computer host 100, or (as shown in FIG. 1)remotely from computer host 100. Embodiments of VM management server 150provide an interface (such as a graphical user interface (or GUI))through which a system administrator may define, configure, and deployvirtual machines for execution on one or more host computers.

In addition, VM management server 150 provides for the configuration ofvirtual machines as highly latency sensitive virtual machines. Accordingto one or more embodiments, VM management server 150 maintains a latencysensitivity table 155, which defines latency sensitivity characteristicsof virtual machines. Latency sensitivity table 155 is described infurther detail below.

As shown in FIG. 1, VM management server 150 communicates with computerhost 100, either through a direct local connection or over a computernetwork. In order to facilitate such communication, VM management agent134 executes on computer host 100. Although VM management agent 134 isnot part of kernel 136, embodiments of the agent run at the hypervisorlevel within hypervisor 130. However, in other embodiments, VMmanagement agent 134 may run as a user program within execution space120. It should be noted that the processes of VM management agent 134are also scheduled for execution by kernel scheduler 135.

VM management agent 134 receives instructions from VM management server150 and carries out tasks on behalf of VM management server 150. Amongthe tasks performed by VM management agent 134 are configuration andinstantiation of virtual machines. One aspect of the configuration of avirtual machine is whether that virtual machine is highly latencysensitive. Thus, VM management agent 134 receives a copy of latencysensitivity table 155 and saves the underlying data within RAM 141 aslatency sensitivity data 143. As shown in FIG. 1, once latencysensitivity data 143 is saved to RAM, kernel scheduler 135 then accessesthat information in order to determine which virtual machines are highlylatency sensitive. Upon determining that one or more virtual machinesare highly latency sensitive, kernel scheduler 135 makes certainscheduling changes in order to accommodate the needs of these virtualmachines.

FIG. 2 is a block diagram that depicts one embodiment of latencysensitivity table 155. As shown in the figure, latency sensitivity table155 stores multiple rows of data, where each row corresponds to aparticular virtual machine within host 100. Each virtual machine isidentified on the host by a unique VM ID 210. A VM ID 210 may be anyunique binary or alphanumeric value that is associated with a virtualmachine. As shown in FIG. 2, latency sensitivity table 155 has entriesfor VM IDs 210 ₁ and 210 ₂, which correspond, respectively, to thevirtual machines VM 110 ₁ and VM 110 ₂ depicted in FIG. 1.

For each VM ID 210, latency sensitivity table 155 stores two values. Afirst value is a latency sensitivity indicator. This indicator may takeon two distinct values (such as Y or N), which indicates whether thecorresponding virtual machine is highly latency sensitive. In otherembodiments, the latency sensitive indicator may take on more than twovalues (e.g., High, Medium, Low, or Normal), to provide for specifyingdifferent degrees of latency sensitivity for the corresponding virtualmachine. In FIG. 2, VM ID 210 ₁ (corresponding to VM 110 ₁) identifies avirtual machine that is not latency sensitive because its latencysensitivity indicator is set to N. On the other hand, VM ID 210 ₂ (whichcorresponds to VM 110 ₂) identifies a virtual machine that is latencysensitive because its corresponding latency sensitivity indicator is setto Y. For example, VM 110 ₂ might be a virtual machine that runs areal-time financial trading application, while VM 110 ₁ might be avirtual machine that runs a batch processing application (such as abilling system), which typically does not require split-second responsetime, and is generally unaffected by the jitter that might occur in avirtualized computing environment.

The second value that latency sensitivity table 155 stores is an“entitlement” value. The CPU resource entitlement for a virtual machinemay be adjusted by specifying a CPU reservation value, a CPU limitvalue, and a CPU shares value. A CPU reservation value represents aguaranteed minimum allocation of CPU resources for the virtual machine.By contrast, a CPU limit value represents a maximum allocation of CPUresources for the virtual machine. Finally, a CPU shares valuerepresents an amount of CPU resources that a virtual machine isallocated relative to an amount of CPU resources allocated to othervirtual machines. Thus, with a CPU shares allocation, the CPUentitlement for a virtual machine may change dynamically as othervirtual machines are powered on, powered off, or have their own CPUshares values configured.

In the embodiment shown in FIG. 2, the entitlement value is apercentage. In this embodiment, the entitlement value represents areserved percentage of CPU resources for the virtual CPUs of a given VM(i.e., a CPU reservation value). That is, the reservation represented byan entitlement value specifies a guaranteed minimum allocation for thecorresponding virtual machine. According to embodiments, a virtualmachine may be powered on only if there are enough unreserved CPUresources available to satisfy the reservation of the virtual machine.It should be noted that the entitlement value is not limited to beingexpressed as a percentage. In other embodiments, the entitlement valuemay be represented as a number of reserved CPU cycles (such as a certainfraction of a processor's speed in gigahertz (GHz)).

In FIG. 2, VM 110 ₁ (as identified by VM ID 210 ₁) has an entitlementvalue of 40 percent (%), which means that 40% of a physical CPU isreserved to the VCPU (i.e., VCPU 125 ₁) associated with VM 110 ₁. Itshould be noted that, in this example, it is assumed that VM 110 ₁ onlyhas a single VCP 125. If, however, VM 110 ₁ has more than one VCPU, theneach of those VCPUs would share 40% of the reserved capacity of a singlephysical CPU.

Referring to FIG. 2, VM 110 ₂ (as identified by VM ID 210 ₂) has acorresponding entitlement value of 100%. Thus, the host will reserve100% of the capacity of a physical CPU for VM 110 ₂.

According to one or more embodiments, if a virtual machine is bothhighly latency sensitive and has a maximum entitlement value, then thatvirtual machine is granted exclusive affinity to one or more physicalCPUs. For example, assuming that VM 110 ₂ has one virtual CPU VCPU 125₂, then VM 110 ₂ (or, more specifically, VCPU 125 ₂) is given exclusiveaffinity to one physical CPU in computer host 100 (i.e., one of PCPUs145 ₁-145 _(M)). On the other hand, if VM 110 ₂ has two virtual CPUs,then, in order for both virtual CPUs to be given exclusive affinity to aphysical CPU, then the entitlement value in latency sensitivity table155 for VM ID 210 ₂ is set to 200% (i.e., 100% reservation for eachvirtual CPU in the set of VCPUs 125 ₂ corresponding to VM 110 ₂).

When a virtual CPU of a virtual machine has exclusive affinity to aphysical CPU, the physical CPU is, effectively, dedicated to runningthat particular virtual CPU. That is, the kernel scheduler (i.e., kernelscheduler 135) will refrain from scheduling any processes for executionon the physical CPU to which the virtual CPU has exclusive affinity,even in cases where the physical CPU is in an idle state. Further,kernel scheduler 135 will refrain from scheduling most interruptprocessing on the physical CPU. Instead, interrupt processing isscheduled and dispatched on other physical CPUs, provided no virtualCPUs have exclusive affinity to such other physical CPUs.

In addition, when an executing virtual machine has its latencysensitivity indicator dynamically set to Y and its entitlementdynamically set to a maximum percentage value, kernel scheduler 135detects (or is informed of) these configuration changes. Further, kernelscheduler 135 detects (or is informed) when a virtual machine having itslatency sensitivity indicator previously set to Y and its entitlementpreviously set to a maximum percentage is powered on. In either case,kernel scheduler 135 takes steps to allocate the required number ofphysical CPUs to the virtual machine's virtual CPUs. Thus, if allphysical CPUs are executing tasks at the time the virtual machine ispowered on (or, alternatively, at the time the latency sensitivityindicator and entitlement of the virtual machine are changed), kernelscheduler 135 migrates an executing task from one physical CPU toanother physical CPU. Kernel scheduler performs this migration for asmany physical CPUs as the number of virtual CPUs of the virtual machine.Thus, when the virtual machine's virtual CPUs have tasks that need to beexecuted, the tasks are executed directly on the allocated physical CPUswithout incurring any scheduling or context-switching overhead. Inaddition, according to embodiments, kernel scheduler 135 ensures that aminimum number of physical CPUs are available for processing the tasksfor non-highly latency sensitive virtual machines. In these embodiments,if powering on and allocating a number of physical CPUs to a highlylatency sensitive virtual machine would result in the number of physicalCPUs available for processing non-highly latency sensitive tasks to fallbelow this minimum number, then the virtual machine is not powered on.

FIG. 3 is a diagram that depicts the processing of a halt instructionissued by a highly latency sensitive virtual machine, according to oneor more embodiments. As previously mentioned, embodiments of the guestoperating systems (such as guest OS 112 ₁ and guest OS 112 ₂) areconfigured to detect whether the virtual machine has additional tasks torun. If the virtual machine has no additional tasks to run, then a guestoperating system in a virtual machine may issue a “virtual” haltinstruction. That is, the guest operating system may issue a halt (suchas the HLT or MWAIT) instruction to one or more virtual CPUs of thevirtual machine. This is done in order to conserve power when thevirtual machine is not performing any useful tasks.

When the guest operating system of a virtual machine issues a haltinstruction to a virtual CPU, the instruction is received and processedby the corresponding VMM. When the virtual machine is not latencysensitive, then the VMM instructs the kernel to deallocate any physicalCPUs previously allocated to executing the halted virtual CPU. Thisscenario is depicted in FIG. 3, where guest OS 112 ₁ running in VM 110 ₁issues a halt instruction (denoted by 310 ₁) to halt VCPU 125 ₁. Itshould be noted that, based on information stored in latency sensitivitytable 155 depicted in FIG. 2, VM 110 ₁ is not a highly latency sensitivevirtual machine. This is due to the fact that the latency sensitivityindicator for VM ID 210 ₁ (which corresponds to VM 110 ₁) is set to N,and because the entitlement value for VM ID 210 ₁ is not a maximum value(e.g., 100%). Thus, VMM 131 ₁ receives the halt instruction and, inresponse, makes a kernel call (denoted in FIG. 3 as 315) to communicateto kernel scheduler 135 that VCPU 125 ₁ is idle.

In response to kernel call 315, kernel scheduler 135 deallocates fromVCPU 125 ₁ any physical CPUs on which VCPU 125 ₁ is currently executing.This process is denoted in FIG. 3 by arrow 320. For example, assumethat, in FIG. 2, task 300 ₁ is a task that is currently executing onPCPU 145 ₁. Further, task 300 ₁ corresponds to VCPU 125 ₁. That is, task300 ₁ represents the execution of VCPU 125 ₁ on physical CPU PCPU 145 ₁.In response to kernel call 315, kernel scheduler 135 deallocates task300 ₁ from executing on PCPU 145 ₁ (which is denoted by the dottedline). This frees PCPU 145 ₁ to execute other tasks that require CPUresources. As shown in FIG. 3, task 300 ₂ is selected by kernelscheduler 135 to execute on PCPU 145 ₁. It should be noted that theprocessing of virtual halt request 310 ₁, kernel call 315, and thescheduling task 320 each consumes physical CPU resources, and thuscontributes to latency. Further, when VCPU 125 ₁ becomes ready toexecute a task (by, for example, receiving a soft interrupt), then theVCPU needs to be rescheduled to execute on a physical CPU by kernelscheduler 135, which is a further source of latency.

In contrast with VM 110 ₁, VM 110 ₂ is a highly latency sensitivevirtual machine. This is due to the fact that the latency sensitiveindicator for VM ID 210 ₂ (which corresponds to VM 110 ₂) is set to Y,and because the entitlement value for VM ID 210 ₂ is set to a maximumvalue (i.e., 100%). In this case, when guest OS 112 ₂ issues a haltinstruction (i.e., an HLT, MWAIT, or equivalent instruction) to VCPU 125₂, VMM 131 ₂ intercepts the instruction. VMM 131 ₂ then determines thatVM 110 ₂ is highly latency sensitive and, therefore, executes in a statewhere its corresponding VCPUs have exclusive affinity to one or morephysical CPUs of computer host 100. For example, in FIG. 3, VMM 131 ₂determines that VCPU 125 ₂ executes with exclusive affinity to PCPU 145₂. Therefore, in order to effectuate the halt of VCPU 125 ₂, VMM 131 ₂issues a physical halt instruction (e.g., a physical HLT, MWAIT, orequivalent instruction) to physical CPU PCPU 145 ₂. It should be notedthat, in the case of latency sensitive VM 110 ₂, VMM 131 ₂ does not makea kernel call to communicate that VCPU 125 ₂ is idle. Rather, the haltinstruction is issued directly to PCPU 145 ₂, which has the effect ofhalting task 300 ₃ (which corresponds to VCPU 125 ₂, and is shown asexecuting on PCPU 145 ₂) without deallocating PCPU 145 ₂ from that task.Thus, when VCPU 125 ₂ receives an interrupt subsequent to receiving thehalt instruction, PCPU 145 ₂ is able to immediately begin processing forVCPU 125 ₂, without any need for kernel scheduler 135 to schedule anycorresponding tasks. Thus, the latency that is present for non-latencysensitive VM 110 ₁ is absent for latency sensitive VM 110 ₂.

FIG. 4 is a flow diagram depicting a method 400 for halting a virtualCPU of a highly latency sensitive virtual machine, according to one ormore embodiments. As shown, the steps of method 400 are executed by avirtual machine monitor (such as VMM 131 ₁ or VMM 131 ₂, depicted inFIG. 3) and by a kernel scheduler (such as kernel scheduler 135). Method400 begins at step 410, where a VMM for a virtual machine receives ahalt instruction for a virtual CPU of the virtual machine. As previouslymentioned, the halt instruction may be the HLT or MWAIT instructions ofthe x86 processor architecture, or any equivalent instruction that haltsexecution of the processor.

Next, at step 420, the VMM determines whether the VMM corresponds to avirtual machine that is highly latency sensitive. According to one ormore embodiments, the VMM determines the latency sensitivity of thecorresponding virtual machine by examining an entry in a latencysensitivity data structure (such as, for example, latency sensitivitydata 143, which is based on latency sensitivity table 155). If the VMMdetermines that the virtual machine that issued the received haltinstruction is highly latency sensitive, then method 400 proceeds tostep 460.

At step 460, the VMM issues a halt instruction to the virtual CPU, whichhas exclusive affinity to one of the physical CPUs of the host computer.Thus, the halt instruction issued to the virtual CPU has the effect ofdirectly halting the physical CPU on which the halted virtual CPUexecutes. Further, the halting of the physical CPU is performed withoutissuing a kernel call. After step 460, method 400 terminates.

If, however, the VMM determines, at step 420, that the virtual machinethat issued the halt instruction for the virtual CPU is not highlylatency sensitive, then method 400 proceeds to step 430. At step 430,the VMM issues a kernel call that informs the kernel scheduler (such askernel scheduler 135 depicted in FIG. 1) that the task that correspondsto the virtual CPU is now idle. It should be noted that, from thestandpoint of the hypervisor and the kernel scheduler, a virtual CPU isa task to be executed, rather than a physical processor.

In response to the kernel call made by the VMM at step 430, the kernelscheduler, at step 440, preempts the task that corresponds to thevirtual CPU. That is, the kernel scheduler halts the task thatcorresponds to the virtual CPU. According to embodiments, the kernelscheduler saves the state of the virtual CPU task and queues that taskfor future execution at such time that the virtual CPU is not in an idlestate.

Next, at step 450, the kernel scheduler schedules for execution anothertask on the physical CPU from which the virtual CPU task was preemptedfrom execution on. This other scheduled task may be another virtual CPUthat is not idle, a user program, an operating system process, or aninterrupt handler. Once the other task is scheduled for execution on thephysical CPU, method 400 terminates.

Although one or more embodiments have been described herein in somedetail for clarity of understanding, it should be recognized thatcertain changes and modifications may be made without departing from thespirit of the disclosure. The various embodiments described herein mayemploy various computer-implemented operations involving data stored incomputer systems. For example, these operations may require physicalmanipulation of physical quantities—usually, though not necessarily,these quantities may take the form of electrical or magnetic signals,where they or representations of them are capable of being stored,transferred, combined, compared, or otherwise manipulated. Further, suchmanipulations are often referred to in terms, such as producing,yielding, identifying, determining, or comparing. Any operationsdescribed herein that form part of one or more embodiments of thedisclosure may be useful machine operations. In addition, one or moreembodiments of the disclosure also relate to a device or an apparatusfor performing these operations. The apparatus may be speciallyconstructed for specific required purposes, or it may be a generalpurpose computer selectively activated or configured by a computerprogram stored in the computer. In particular, various general purposemachines may be used with computer programs written in accordance withthe teachings herein, or it may be more convenient to construct a morespecialized apparatus to perform the required operations.

The various embodiments described herein may be practiced with othercomputer system configurations including hand-held devices,microprocessor systems, microprocessor-based or programmable consumerelectronics, minicomputers, mainframe computers, and the like.

One or more embodiments of the present disclosure may be implemented asone or more computer programs or as one or more computer program modulesembodied in one or more computer readable media. The term computerreadable medium refers to any data storage device that can store datawhich can thereafter be input to a computer system—computer readablemedia may be based on any existing or subsequently developed technologyfor embodying computer programs in a manner that enables them to be readby a computer. Examples of a computer readable medium include a harddrive, network attached storage (NAS), read-only memory, random-accessmemory (e.g., a flash memory device), a CD (Compact Discs) —CD-ROM, aCD-R, or a CD-RW, a DVD (Digital Versatile Disc), a magnetic tape, andother optical and non-optical data storage devices. The computerreadable medium can also be distributed over a network coupled computersystem so that the computer readable code is stored and executed in adistributed fashion.

Although one or more embodiments of the present disclosure have beendescribed in some detail for clarity of understanding, it will beapparent that certain changes and modifications may be made within thescope of the claims. Accordingly, the described embodiments are to beconsidered as illustrative and not restrictive, and the scope of theclaims is not to be limited to details given herein, but may be modifiedwithin the scope and equivalents of the claims. In the claims, elementsand/or steps do not imply any particular order of operation, unlessexplicitly stated in the claims.

Many variations, modifications, additions, and improvements arepossible. Plural instances may be provided for components, operations orstructures described herein as a single instance. Boundaries betweenvarious components, operations and data stores are somewhat arbitrary,and particular operations are illustrated in the context of specificillustrative configurations. Other allocations of functionality areenvisioned and may fall within the scope of the disclosure(s). Ingeneral, structures and functionality presented as separate componentsin exemplary configurations may be implemented as a combined structureor component. Similarly, structures and functionality presented as asingle component may be implemented as separate components. These andother variations, modifications, additions, and improvements may fallwithin the scope of the appended claim(s).

We claim:
 1. In a host computer having a virtualization software thatsupports execution of a plurality of virtual machines, thevirtualization software including a virtual machine monitor for each ofthe virtual machines, wherein each virtual machine monitor emulates avirtual central processing unit (CPU) for a corresponding virtualmachine, a method of halting execution of a virtual CPU of a virtualmachine, the method comprising: receiving a first halt instruction at avirtual machine monitor from a corresponding virtual machine;determining by the virtual machine monitor whether the virtual machineis latency sensitive; if the virtual machine is latency sensitive, thenissuing from the virtual machine monitor a second halt instruction tohalt a physical CPU on which the virtual CPU executes; and if thevirtual machine is not latency sensitive, then executing a system callto a kernel executing on the host computer to indicate to the kernelthat the virtual CPU is in an idle state.
 2. The method of claim 1,wherein the first halt instruction is issued by a guest operating systemexecuting in the virtual machine.
 3. The method of claim 1, wherein, ifthe virtual machine is latency sensitive, then the virtual CPU executesin an exclusive mode on the physical CPU.
 4. The method of claim 1,wherein determining whether the virtual machine is latency sensitivecomprises: reading a latency sensitivity indicator for the virtualmachine; reading a CPU entitlement value for the virtual machine;determining whether the latency sensitivity indicator is a predeterminedvalue; and determining whether the CPU entitlement value is a maximumvalue.
 5. The method of claim 1, wherein the system call is made to akernel CPU scheduler.
 6. The method of claim 5, wherein the kernel CPUscheduler, in response to the system call, deschedules the virtual CPU.7. The method of claim 6, wherein, in response to the system call, thekernel CPU scheduler deallocates the physical CPU from the virtual CPUand allocates the physical CPU to other tasks.
 8. A non-transitorycomputer-readable medium comprising instructions executable by a hostcomputer, the host computer having a virtualization software thatsupports execution of a plurality of virtual machines, thevirtualization software including a virtual machine monitor for each ofthe virtual machines, wherein each virtual machine monitor emulates avirtual central processing unit (CPU) for a corresponding virtualmachine, where the instructions, when executed, cause the host computerto perform method of halting execution of a virtual CPU of a virtualmachine, the method comprising: receiving a first halt instruction at avirtual machine monitor from a corresponding virtual machine;determining by the virtual machine monitor whether the virtual machineis latency sensitive; if the virtual machine is latency sensitive, thenissuing from the virtual machine monitor a second halt instruction tohalt a physical CPU on which the virtual CPU executes; and if thevirtual machine is not latency sensitive, then executing a system callto a kernel executing on the host computer to indicate to the kernelthat the virtual CPU is in an idle state.
 9. The computer-readablemedium of claim 8, wherein the first halt instruction is issued by aguest operating system executing in the virtual machine.
 10. Thecomputer-readable medium of claim 8, wherein, if the virtual machine islatency sensitive, then the virtual CPU executes in an exclusive mode onthe physical CPU.
 11. The computer-readable medium of claim 8, whereindetermining whether the virtual machine is latency sensitive comprises:reading a latency sensitivity indicator for the virtual machine; readinga CPU entitlement value for the virtual machine determining whether thelatency sensitivity indicator is a predetermined value; and determiningwhether the CPU entitlement value is a maximum value.
 12. Thecomputer-readable medium of claim 8, wherein the system call is made toa kernel CPU scheduler.
 13. The computer-readable medium of claim 12,wherein the kernel CPU scheduler, in response to the system call,deschedules the virtual CPU.
 14. The computer-readable medium of claim13, wherein, in response to the system call, the kernel CPU schedulerdeallocates the physical CPU from the virtual CPU and allocates thephysical CPU to other tasks.
 15. A virtualized computing system,comprising: a host computer having a virtualization software thatsupports execution of a plurality of virtual machines; a kernelscheduler; and a virtual machine monitor for each of the virtualmachines, wherein each virtual machine monitor emulates a virtualcentral processing unit (CPU) for a corresponding virtual machine,wherein each virtual machine monitor is configured to perform a methodof halting execution of a virtual CPU of a virtual machine, the methodcomprising: receiving a first halt instruction at a virtual machinemonitor from a corresponding virtual machine; determining by the virtualmachine monitor whether the virtual machine is latency sensitive; if thevirtual machine is latency sensitive, then issuing from the virtualmachine monitor a second halt instruction to halt a physical CPU onwhich the virtual CPU executes; and if the virtual machine is notlatency sensitive, then executing a system call to a kernel executing onthe host computer to indicate to the kernel that the virtual CPU is inan idle state.
 16. The system of claim 15, wherein the first haltinstruction is issued by a guest operating system executing in thevirtual machine.
 17. The system of claim 15, wherein, if the virtualmachine is latency sensitive, then the virtual CPU executes in anexclusive mode on the physical CPU.
 18. The system of claim 15, whereindetermining whether the virtual machine is latency sensitive comprises:reading a latency sensitivity indicator for the virtual machine; readinga CPU entitlement value for the virtual machine determining whether thelatency sensitivity indicator is a predetermined value; and determiningwhether the CPU entitlement value is a maximum value.
 19. The system ofclaim 15, wherein the system call is made to a kernel CPU scheduler. 20.The system of claim 19, wherein the kernel CPU scheduler, in response tothe system call, deschedules the virtual CPU, deallocates the physicalCPU from the virtual CPU, and allocates the physical CPU to other tasks.